In typical cellular wireless communications, each end-user device (e.g., cell phone, personal digital assistant, wirelessly equipped personal computer, etc.) subscribes to service from a given cellular wireless carrier known as the device's home carrier. In practice, the home carrier will operate one or more radio access networks including base stations that radiate to define wireless coverage areas in which the end-user devices can operate. When a device enters into coverage of its home carrier's network, the device may register with the home carrier network and may then engage in wireless communication service via the home carrier's network, and the home carrier may accordingly charge or debit a service account of the device.
Although a typical cellular carrier may strive to offer wireless coverage throughout a region, such as nationally in the United States for instance, the carrier may not actually operate radio access networks in all locations throughout the region. To fill in gaps where the carrier does not operate its own radio access networks and thus where the carrier does not itself provide coverage, the carrier will typically enter into roaming agreements with other carriers. Under a roaming agreement, another carrier may agree to provide service to the home carrier's subscriber devices when the devices operate in the roaming carrier's network. Thus, when a device enters into coverage of a roaming carrier's network, the device may register for service with the roaming carrier's network and may then engage in wireless communication service via the roaming carrier's network. In accordance with the roaming agreement, the roaming carrier may then report that usage to the home carrier and may charge the home carrier for the usage. In turn, the home carrier may then pass that charge along to the service account of the device, or, for certain users, the home carrier may absorb the charge.
As a general matter, a home carrier may prefer to have its subscribers operate in coverage of the home carrier's network, rather than in coverage of a roaming carrier's network. One reason for this is that the home carrier will typically profit more when the home carrier charges the subscriber for use of the home carrier's network and services than when the home carrier merely passes a roaming carrier's charges along to the subscriber or absorbs the roaming charges. Another reason for this is that, when a subscriber is roaming, the subscriber may not benefit from full access to the home carrier's services and support, which may lead to poor user experience.
The choice of operating in home network coverage or roaming network coverage becomes an issue in a region where both home network coverage and roaming network coverage are provided. Throughout such a region, there may be some areas where the home carrier provides coverage and other areas where the home carrier does not provide coverage but a roaming carrier provides coverage, and there may be areas where both the home carrier and a roaming carrier provide coverage.
To promote subscriber operation in home network coverage, the home carrier will typically provision each of its subscriber devices with a “preferred roaming list” (PRL) that defines a priority order for each device to scan for available network coverage. The PRL will generally give higher priority to networks or systems operated by the home carrier than to networks or systems operated by roaming carriers, and the PRL may further define a priority order among roaming carriers. As the device scans for available coverage according to the PRL, the device will thus typically first scan for coverage of a home carrier network and, if the device does not succeed, will then scan for coverage of a roaming carrier network. In either case, once the device finds a suitable network, the device may then register for service with that network and then proceed to engage in communication service via that network.
In practice, when a subscriber device is not engaged in an active communication session (e.g., a voice call or data-session), the device may periodically scan the airwaves in search of a most preferred coverage area according to its PRL. Once the device finds such a coverage area, the device may then “idle” on the coverage area. In typical idle mode, the device will periodically scan air interface control channels of the coverage area in order to receive any relevant page messages. Further, if the device seeks to originate (initiate) a new communication session, the device would do so in the coverage area where it has been idling.
The rate or period at which a device scans for a more preferred system may be defined by a rescan timer (sometimes referred to as a “best system rescanning timer” or “BSR timer”) programmed in the device. The rescan interval may be established based on a consideration of two competing factors. On the one hand, if the device is battery powered, then it would be best to use a relatively long rescan interval, since each rescan by the device consumes battery power. On the other hand, if the rescan interval is relatively long, then there is a greater likelihood that when the device originates a new communication, the device will do so in a system that is not the most preferred currently available, such as in a roaming system at a location where home coverage is just as well available but has not yet been detected by the device. By engineering design, a suitable rescan interval (e.g., 180 seconds) is therefore typically chosen and programmed in subscriber devices.
When a subscriber device is operating on its home system and moves to a border between home coverage and roaming coverage (e.g., in an area of overlap between the two), the device may begin to idle on the roaming system due to a loss of adequate home coverage. Once the device begins roaming, the device may then originate any new communications on the roaming system as noted above. Further, the device may not move back to idle on home system until the device performs a next BSR scan, and only then if the device thereby determines that home coverage is once again available.